Is it possible to make GC manage native object's lifetime?

Is it possible to make GC manage native object's lifetime? - java

With C++ and C# experience and some little Java knowledge I'm now starting a Java+JNI (C++) project (Android, if that matters).
I have a native method, that creates some C++ class and returns a pointer to it as a Java long value (say, handle). And then other native methods called from Java code here and there, use the handle as a parameter to do some native operations on this class. C++ side does not own the object, it's Java side who does. But in the current architecture design it's hard to define who exactly owns the object and when to delete it. So it would probably be nice to make Java VM garbage collector to manage the object's lifetime somehow. The C++ class does not consume any resources, except some piece of memory, not large. So it's OK, if several such objects will not be destructed.
In C# I would probably wrap the native IntPtr handle in some managed wrapper class. And override it's finalizer to call native object's destructor when the managed wrapper is garbage collected. SafeHandle, AddMemoryPressure, etc. might be also of help here.
This is a different story with Java's finalize. The second thing you know after 'Hello world' in Java, is that using finalize is bad. Are there any other ways to accomplish this in Java? Maybe using PhantomReference?

Well let's consider the reason WHY finalize and Co are problematic: As you know there's no guarantee that the finalize will be called before the VM is shut down, which means that special cleanup code won't necessarily run (imo a bad decision, I don't see any problems to run through the finalize queue at cleanup, but well that's how it is). Also this is exactly the same situation in C#
Now your objects only consume memory, which will be cleaned up by the OS anyhow when the VM is destroyed, so the only case where finalize is problematic won't matter for you. So yes you can indeed use this variant and it'll work perfectly fine, but it may not exactly be considered a great architectural design - and as soon as you add resources to your C++ code where the OS doesn't handle the cleanup correctly you will run into problems
Also note that implementing a finalizer results in some additional overhead for the GC and means it takes two cycles to cleanup one of these objects (and whatever you do, don't ever save an object in the finalize method)

If you understand why you should avoid using Java's finalize method, you will also understand how to use it correctly. Using finalize for closing system resources (files and handles) is bad because you don't actually know when those resources will be closed and released. Using complex finalize logic is bad as your object reference can leak out and get pinned in memory again.
For your scenario, it is perfectly fine to use finalize.

using a wrapper with a finalizer is a decent solution here
but if you really don't wanna do that you can use a PhantomReference with a ReferenceQueue to clean it up (but you are going to require a separate thread to poll the queue)

So how can we achieve it using phantom reference.
Create a wrapper object for your native intPtr object. Create a
phantom reference(with a reference queue) on the wrapper object.
Create and maintain a map of phantom reference to intPtr.
Create a thread that will be monitoring the reference queue for finalized
wrapper object instances.
This thread will get the phantom reference from reference queue, lookup intPtr using phantom reference and call destructor on native int object referenced by intPtr.
While all this happening, you can go about happily using the
wrapper object in your java code.

Related

Is it possible to run code when the object in WeakReference is garbage collected? [duplicate]

Is there a destructor for Java? I don't seem to be able to find any documentation on this. If there isn't, how can I achieve the same effect?
To make my question more specific, I am writing an application that deals with data and the specification say that there should be a 'reset' button that brings the application back to its original just launched state. However, all data have to be 'live' unless the application is closed or reset button is pressed.
Being usually a C/C++ programmer, I thought this would be trivial to implement. (And hence I planned to implement it last.) I structured my program such that all the 'reset-able' objects would be in the same class so that I can just destroy all 'live' objects when a reset button is pressed.
I was thinking if all I did was just to dereference the data and wait for the garbage collector to collect them, wouldn't there be a memory leak if my user repeatedly entered data and pressed the reset button? I was also thinking since Java is quite mature as a language, there should be a way to prevent this from happening or gracefully tackle this.

Because Java is a garbage collected language you cannot predict when (or even if) an object will be destroyed. Hence there is no direct equivalent of a destructor.
There is an inherited method called finalize, but this is called entirely at the discretion of the garbage collector. So for classes that need to explicitly tidy up, the convention is to define a close method and use finalize only for sanity checking (i.e. if close has not been called do it now and log an error).
There was a question that spawned in-depth discussion of finalize recently, so that should provide more depth if required...

Have a look at the try-with-resources statement. For example:
try (BufferedReader br = new BufferedReader(new FileReader(path))) {
System.out.println(br.readLine());
} catch (Exception e) {
...
} finally {
...
}
Here the resource that is no longer needed is freed in the BufferedReader.close() method. You can create your own class that implements AutoCloseable and use it in a similar fashion.
This statement is more limited than finalize in terms of code structuring, but at the same time it makes the code simpler to understand and maintain. Also, there is no guarantee that a finalize method is called at all during the livetime of the application.

Nope, no destructors here. The reason is that all Java objects are heap allocated and garbage collected. Without explicit deallocation (i.e. C++'s delete operator) there is no sensible way to implement real destructors.
Java does support finalizers, but they are meant to be used only as a safeguard for objects holding a handle to native resources like sockets, file handles, window handles, etc. When the garbage collector collects an object without a finalizer it simply marks the memory region as free and that's it. When the object has a finalizer, it's first copied into a temporary location (remember, we're garbage collecting here), then it's enqueued into a waiting-to-be-finalized queue and then a Finalizer thread polls the queue with very low priority and runs the finalizer.
When the application exits, the JVM stops without waiting for the pending objects to be finalized, so there practically no guarantees that your finalizers will ever run.

Use of finalize() methods should be avoided. They are not a reliable mechanism for resource clean up and it is possible to cause problems in the garbage collector by abusing them.
If you require a deallocation call in your object, say to release resources, use an explicit method call. This convention can be seen in existing APIs (e.g. Closeable, Graphics.dispose(), Widget.dispose()) and is usually called via try/finally.
Resource r = new Resource();
try {
//work
} finally {
r.dispose();
}
Attempts to use a disposed object should throw a runtime exception (see IllegalStateException).
EDIT:
I was thinking, if all I did was just
to dereference the data and wait for
the garbage collector to collect them,
wouldn't there be a memory leak if my
user repeatedly entered data and
pressed the reset button?
Generally, all you need to do is dereference the objects - at least, this is the way it is supposed to work. If you are worried about garbage collection, check out Java SE 6 HotSpot[tm] Virtual Machine Garbage Collection Tuning (or the equivalent document for your JVM version).

With Java 1.7 released, you now have the additional option of using the try-with-resources block. For example,
public class Closeable implements AutoCloseable {
#Override
public void close() {
System.out.println("closing...");
}
public static void main(String[] args) {
try (Closeable c = new Closeable()) {
System.out.println("trying...");
throw new Exception("throwing...");
}
catch (Exception e) {
System.out.println("catching...");
}
finally {
System.out.println("finalizing...");
}
}
}
If you execute this class, c.close() will be executed when the try block is left, and before the catch and finally blocks are executed. Unlike in the case of the finalize() method, close() is guaranteed to be executed. However, there is no need of executing it explicitly in the finally clause.

I fully agree to other answers, saying not to rely on the execution of finalize.
In addition to try-catch-finally blocks, you may use Runtime#addShutdownHook (introduced in Java 1.3) to perform final cleanups in your program.
That isn't the same as destructors are, but one may implement a shutdown hook having listener objects registered on which cleanup methods (close persistent database connections, remove file locks, and so on) can be invoked - things that would normally be done in destructors.
Again - this is not a replacement for destructors but in some cases, you can approach the wanted functionality with this.
The advantage of this is having deconstruction behavior loosely coupled from the rest of your program.

No, java.lang.Object#finalize is the closest you can get.
However, when (and if) it is called, is not guaranteed.
See: java.lang.Runtime#runFinalizersOnExit(boolean)

I agree with most of the answers.
You should not depend fully on either finalize or ShutdownHook
finalize
The JVM does not guarantee when this finalize() method will be invoked.
finalize() gets called only once by GC thread. If an object revives itself from finalizing method, then finalize will not be called again.
In your application, you may have some live objects, on which garbage collection is never invoked.
Any Exception that is thrown by the finalizing method is ignored by the GC thread
System.runFinalization(true) and Runtime.getRuntime().runFinalization(true) methods increase the probability of invoking finalize() method but now these two methods have been deprecated. These methods are very dangerous due to lack of thread safety and possible deadlock creation.
shutdownHooks
public void addShutdownHook(Thread hook)
Registers a new virtual-machine shutdown hook.
The Java virtual machine shuts down in response to two kinds of events:
The program exits normally, when the last non-daemon thread exits or when the exit (equivalently, System.exit) method is invoked, or
The virtual machine is terminated in response to a user interrupt, such as typing ^C, or a system-wide event, such as user logoff or system shutdown.
A shutdown hook is simply an initialized but non-started thread. When the virtual machine begins its shutdown sequence it will start all registered shutdown hooks in some unspecified order and let them run concurrently. When all the hooks have finished it will then run all uninvoked finalizers if finalization-on-exit has been enabled.
Finally, the virtual machine will halt. Note that daemon threads will continue to run during the shutdown sequence, as will non-daemon threads if the shutdown was initiated by invoking the exit method.
Shutdown hooks should also finish their work quickly. When a program invokes exit the expectation is that the virtual machine will promptly shut down and exit.
But even Oracle documentation quoted that
In rare circumstances the virtual machine may abort, that is, stop running without shutting down cleanly
This occurs when the virtual machine is terminated externally, for example with the SIGKILL signal on Unix or the TerminateProcess call on Microsoft Windows. The virtual machine may also abort if a native method goes awry by, for example, corrupting internal data structures or attempting to access nonexistent memory. If the virtual machine aborts then no guarantee can be made about whether or not any shutdown hooks will be run.
Conclusion : use try{} catch{} finally{} blocks appropriately and release critical resources in finally(} block. During release of resources in finally{} block, catch Exception and Throwable.

First, note that since Java is garbage-collected, it is rare to need to do anything about object destruction. Firstly because you don't usually have any managed resources to free, and secondly because you can't predict when or if it will happen, so it's inappropriate for things that you need to occur "as soon as nobody is using my object any more".
You can be notified after an object has been destroyed using java.lang.ref.PhantomReference (actually, saying it has been destroyed may be slightly inaccurate, but if a phantom reference to it is queued then it's no longer recoverable, which usually amounts to the same thing). A common use is:
Separate out the resource(s) in your class that need to be destructed into another helper object (note that if all you're doing is closing a connection, which is a common case, you don't need to write a new class: the connection to be closed would be the "helper object" in that case).
When you create your main object, create also a PhantomReference to it. Either have this refer to the new helper object, or set up a map from PhantomReference objects to their corresponding helper objects.
After the main object is collected, the PhantomReference is queued (or rather it may be queued - like finalizers there is no guarantee it ever will be, for example if the VM exits then it won't wait). Make sure you're processing its queue (either in a special thread or from time to time). Because of the hard reference to the helper object, the helper object has not yet been collected. So do whatever cleanup you like on the helper object, then discard the PhantomReference and the helper will eventually be collected too.
There is also finalize(), which looks like a destructor but doesn't behave like one. It's usually not a good option.

The finalize() function is the destructor.
However, it should not be normally used because it is invoked after the GC and you can't tell when that will happen (if ever).
Moreover, it takes more than one GC to deallocate objects that have finalize().
You should try to clean up in the logical places in your code using the try{...} finally{...} statements!

If it's just memory you are worried about, don't. Just trust the GC it does a decent job. I actually saw something about it being so efficient that it could be better for performance to create heaps of tiny objects than to utilize large arrays in some instances.

Perhaps you can use a try ... finally block to finalize the object in the control flow at which you are using the object. Of course it doesn't happen automatically, but neither does destruction in C++. You often see closing of resources in the finally block.

There is a #Cleanup annotation in Lombok that mostly resembles C++ destructors:
#Cleanup
ResourceClass resource = new ResourceClass();
When processing it (at compilation time), Lombok inserts appropriate try-finally block so that resource.close() is invoked, when execution leaves the scope of the variable. You can also specify explicitly another method for releasing the resource, e.g. resource.dispose():
#Cleanup("dispose")
ResourceClass resource = new ResourceClass();

The closest equivalent to a destructor in Java is the finalize() method. The big difference to a traditional destructor is that you can't be sure when it'll be called, since that's the responsibility of the garbage collector. I'd strongly recommend carefully reading up on this before using it, since your typical RAIA patterns for file handles and so on won't work reliably with finalize().

Just thinking about the original question... which, I think we can conclude from all the other learned answers, and also from Bloch's essential Effective Java, item 7, "Avoid finalizers", seeks the solution to a legitimate question in a manner which is inappropriate to the Java language...:
... wouldn't a pretty obvious solution to do what the OP actually wants be to keep all your objects which need to be reset in a sort of "playpen", to which all other non-resettable objects have references only through some sort of accessor object...
And then when you need to "reset" you disconnect the existing playpen and make a new one: all the web of objects in the playpen is cast adrift, never to return, and one day to be collected by the GC.
If any of these objects are Closeable (or not, but have a close method) you could put them in a Bag in the playpen as they are created (and possibly opened), and the last act of the accessor before cutting off the playpen would be to go through all the Closeables closing them... ?
The code would probably look something like this:
accessor.getPlaypen().closeCloseables();
accessor.setPlaypen( new Playpen() );
closeCloseables would probably be a blocking method, probably involving a latch (e.g. CountdownLatch), to deal with (and wait as appropriate for) any Runnables/Callables in any threads specific to the Playpen to be ended as appropriate, in particular in the JavaFX thread.

Many great answers here, but there is some additional information about why you should avoid using finalize().
If the JVM exits due to System.exit() or Runtime.getRuntime().exit(), finalizers will not be run by default. From Javadoc for Runtime.exit():
The virtual machine's shutdown sequence consists of two phases. In the first phase all registered shutdown hooks, if any, are started in some unspecified order and allowed to run concurrently until they finish. In the second phase all uninvoked finalizers are run if finalization-on-exit has been enabled. Once this is done the virtual machine halts.
You can call System.runFinalization() but it only makes "a best effort to complete all outstanding finalizations" – not a guarantee.
There is a System.runFinalizersOnExit() method, but don't use it – it's unsafe, deprecated long ago.

If you got the chance of using a Contexts and Dependency Injection (CDI) framework such as Weld you can use the Java annotation #Predestroy for doing cleanup jobs etc.
#javax.enterprise.context.ApplicationScoped
public class Foo {
#javax.annotation.PreDestroy
public void cleanup() {
// do your cleanup
}
}

Though there have been considerable advancements in Java's GC technology, you still need to be mindful of your references. Numerous cases of seemingly trivial reference patterns that are actually rats nests under the hood come to mind.
From your post it doesn't sound like you're trying to implement a reset method for the purpose of object reuse (true?). Are your objects holding any other type of resources that need to be cleaned up (i.e., streams that must be closed, any pooled or borrowed objects that must be returned)? If the only thing you're worried about is memory dealloc then I would reconsider my object structure and attempt to verify that my objects are self contained structures that will be cleaned up at GC time.

If you're writing a Java Applet, you can override the Applet "destroy()" method. It is...
* Called by the browser or applet viewer to inform
* this applet that it is being reclaimed and that it should destroy
* any resources that it has allocated. The stop() method
* will always be called before destroy().
Obviously not what you want, but might be what other people are looking for.

No Java doesn't have any destructors .The main reason behind it in Java is the Garbage Collectors that passively works in the background always and all the objects are made in the heap memory , that is the place where GC works .In c++ there we have to explicitly call the delete function since there is no Garbage collector like thing.

In Java, the garbage collector automatically deletes the unused objects to free up the memory. So it’s sensible Java has no destructors available.

Try calling the onDestroy() method when it comes to android programming. This is the last method that executed just before the Activity/Service class is killed.

Missing form all the answers I just scanned is the safer replacement for finalizers. All of the other answers are correct about using try-with-resources and avoiding finalizers as they are unreliable and are now deprecated...
However they haven't mentioned Cleaners. Cleaners were added in Java 9 to explicitly handle the job of cleanup in a better way than finalizers.
https://docs.oracle.com/javase/9/docs/api/java/lang/ref/Cleaner.html

I used to mainly deal with C++ and that is what lead me to the search of a destructor as well. I am using JAVA a lot now. What I did, and it may not be the best case for everyone, but I implemented my own destructor by reseting all the values to either 0 or there default through a function.
Example:
public myDestructor() {
variableA = 0; //INT
variableB = 0.0; //DOUBLE & FLOAT
variableC = "NO NAME ENTERED"; //TEXT & STRING
variableD = false; //BOOL
}
Ideally this won't work for all situations, but where there are global variables it will work as long as you don't have a ton of them.
I know I am not the best Java programmer, but it seems to be working for me.

.NET GC stuck on JNI call from finalizer()

I have a .NET application that is using JNI to call Java code. On the .NET finalizer we call a JNI call to clean the connected resource on Java. But from time to time this JNI gets stuck.
This as expected stuck the all .NET process and never releases.
Bellow you can see the thread dump we got from .NET:
NET Call Stack
Function
.JNIEnv_.NewByteArray(JNIEnv_*, Int32)
Bridge.NetToJava.JVMBridge.ExecutePBSCommand(Byte[], Int32, Byte[])
Bridge.Core.Internal.Pbs.Commands.PbsDispatcher.Execute(Bridge.Core.Internal.Pbs.PbsOutputStream, Bridge.Core.Internal.DispatcherObjectProxy)
Bridge.Core.Internal.Pbs.Commands.PbsCommandsBundle.ExecuteGenericDestructCommand(Byte, Int64, Boolean)
Bridge.Core.Internal.DispatcherObjectProxy.Dispose(Boolean)
Bridge.Core.Internal.Transaction.Dispose(Boolean)
Bridge.Core.Internal.DispatcherObjectProxy.Finalize()
Full Call Stack
Function
ntdll!KiFastSystemCallRet
ntdll!NtWaitForSingleObject+c
kernel32!WaitForSingleObjectEx+ac
kernel32!WaitForSingleObject+12
jvm!JVM_FindSignal+5cc49
jvm!JVM_FindSignal+4d0be
jvm!JVM_FindSignal+4d5fa
jvm!JVM_FindSignal+beb8e
jvm+115b
jvm!JNI_GetCreatedJavaVMs+1d26
Bridge_NetToJava+1220
clr!MethodTable::SetObjCreateDelegate+bd
clr!MethodTable::CallFinalizer+ca
clr!SVR::CallFinalizer+a7
clr!WKS::GCHeap::TraceGCSegments+239
clr!WKS::GCHeap::TraceGCSegments+415
clr!WKS::GCHeap::FinalizerThreadWorker+cd
clr!Thread::DoExtraWorkForFinalizer+114
clr!Thread::ShouldChangeAbortToUnload+101
clr!Thread::ShouldChangeAbortToUnload+399
clr!ManagedThreadBase_NoADTransition+35
clr!ManagedThreadBase::FinalizerBase+f
clr!WKS::GCHeap::FinalizerThreadStart+10c
clr!Thread::intermediateThreadProc+4b
kernel32!BaseThreadStart+34

I have no idea whether .NET finalizers are equally bad idea to Java finalizers, but using a potentially (dead)locking code (i see Win32 condition call at the very bottom) from anything like finalizer (regardless of the platform) is definitely a bad idea. You need to clean your native code of any potential locking, or have an emergency brake timeout at the level of .NET

As I didn't find a question I won't post a formal answer here but rather tell a story about something similar I underwent sometimes:
We created C ojects via JNI, that were backed by java object, and we decided to clean the C objects within the finalize method. However, we envisioned deadlocks, as the finalize is called from a non-application thread, the garbage-collector. As the entire wolrd is stopped while collecting the garbage, whenever the finalizer meets a lock it's immediately a dead lock. Thus we decided to use a java mechnism called phantom references. It's possible to bind a number to each of these 'references' (the C pointer) and then the VM removes an referenced object it puts such an reference into a queue. And one can pull this data whenever appropriate and remove the C object.
I think at least your problem is the same.

android / java - how to be notified when there is no reference to an instance

this question is for either android or java .
suppose i have an instance of a class (even a thread) .
i wish that in order to make it fool proof against forgetting to dispose/close the instance (and avoid any possible memory leaks) , if there are no more references to this instance , it will automatically call a specific method , disposing/closing itself (in case of a thread , it will probably interrupt itself) .
is it possible to enforce such a thing?
if needed , i don't mind that such a thing will occur only during GC .

if there are no more references to this instance , it will
automatically call a specific method , disposing/closing itself (in
case of a thread , it will probably interrupt itself)
finalize() does what you describe here. You very rarely see it used though, and there are some pitfalls when using it. As you cant control garbage collection, you cant be sure when finalize will be run, if ever! From the api:
the usual purpose of finalize, however, is to perform cleanup actions
before the object is irrevocably discarded. For example, the finalize
method for an object that represents an input/output connection might
perform explicit I/O transactions to break the connection before the
object is permanently discarded.
You cant enforce garbage collection. You can only suggest the JVM to do so using System.gc(), but it is not guaranteed that it will be done.

I know this is very late but I hope it might help someone someday.
You can receive such events this by using the using the library that i am developing called gcRadar. It provides events when an object is orphaned and after the actual garbage collection of the object.
Any suggestions for improvements in the library are welcome.

There is no way to access the references held by the VM. As Zavior suggested, the only way to know for sure that an object, or an "island" of objects is inaccessible, is to use the finalize method.
Please note that you will only get notified during GC runs. So it does not really help closing/disposing resources that are still referenced but should be closed. If you want to do that as well and do not want to use such constructs as try/catch/finally, you should write a manager class for the resources.
With any of the possibillities, including a manager class, you will not get a "bulletproof" way to consolidate your resources. Being careful is the best solution IMHO.
EDIT:
I have found this thread that may be useful.

Thread-aware heap allocation tracking with JVMTI

Writing a profiling I would also implement the typical task of heap profiling. Specifically I would like to track, which thread has allocated how much data? Using JVMTI I thought it's sufficient to hook to the events VM Object Allocation and Object Free. Sadly I read the first event is not triggered due to calls made to new.
The last idea I had was to check teh event MethodExit if its name is <init> and thus declare this call as an object allocation. However, within this event I cannot get the object and thus I cannot invoke GetObjectSize.
Simply iterating over the heap, bears no information regarding which object was allocated by which thread. Does anyone have an idea how to implement this?

A quick glance into the _new implementation of the Hotspot VM (templateTable_x86_64.cpp) seems to indicate, that _new doesn't offer any hooks for JVMTI (not even in the slow case it seems). So if your trick doesn't work, I don't see any other possibility - but I'm by no means an expert for JVMTI.
I assume compiling your own Hotspot VM with a small patch isn't especially useful for you?

This heapTracker demo illustrates you how to track all the objects in the heap.
Because the VMObjectAlloc Event is sent only when reflection occurs, the demo uses ByteCodeInstrument to track new object allocation.
You can use getCurrentThread function to know which thread the object is belong to.

Is there some reason you can't call GetObjectSize from the MethodEntry event for a constructor?
If you're interested in executing code before a method returns, then you can listen for the MethodEntry event, and if the method is named <init>, you can call NotifyFramePop to listen for the FramePop event for the current frame. This event is similar to the MethodExit event, but occurs before the method returns so you can still get the this object.

How to detect when an object is no longer referenced

Is there a way to create register a handler that will be called exactly at the time when the last reference to a certain object is released?
An example would be an object that is backed by a physical data file and once the object become unreferenced, the file should be closed and than renamed. It would be nice if this was possible without having to explicitly call a "close" method on that object.
All the notification mechanisms I am aware of from the Weak/Phantom reference area only state that notification will occur at some point in time but there is no gurantee as to when this will happen...

In short, no.
The Java specification explicitly denies you the ability to know when the last reference is released. JVM implementations (and optimizations) depend on this. There is no hook.

From my understanding, and I've looked for some time to find a "destructor" for java objects, there is no way to know when you lose the last reference. Java tracks references to objects but for performance reasons, updates this information only during garbage collection.
The closest thing is the finalize method which should be called during garbage collection but there's no guarantee that it will be called even then.

I think WeakReference does what you want. A WeakReference gets put into the ReferenceQueue as soon as its weakly reachable (i.e. all strong references are gone).
See this article by Ethan Nicholas.
If you are worried about some references not reaching the ReferenceQueue at shutdown, then keep a list of all objects created (using WeakReferences or PhantomReferences). Add a shutdown hook that checks the list for any outstanding references and perform whatever action you need.

Problem is, "How do you implement this, without something holding a reference to the object?"
Even if you could get passed that problem, say with a service we'll call the HandleManager, the HandleManager would then have to create a new reference to the object, to pass to your handler. Then, your handler could either (a) store a reference to it, which would confuse the HandleManager which was expecting to destroy the unreferenced object; or (b) release the reference, which means that the final reference was once again released, which means the Handler has to be called again....

If you need to manage external resources like files, the best you can do in java is a close() function (whatever name you choose). You can use finalize() as a "belt and suspenders" insurance policy, but that has unpredictable timing. So your main line of defense needs to be the close() function.
See my answer Why would you ever implement finalize()?

This cannot be done with Java -- it needs a reference-counting garbage collector as far as I can tell. Have you considered opening and closing your object's physical data file as needed, rather than keeping it open for the lifetime of the object?

You could override finalize() in your object, but that is problematic for reasons others have mentioned.
For your specific example, you could take a look at using something like File.deleteOnExit(), which would remove the file once the VM exits.

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